The present invention relates to a tilt detecting device provided with an electric bubble tube, and in particular, to a tilt detecting device provided with a light transmission type electric bubble tube.
In case an equipment such a survey instrument is to be installed, which must be placed at a horizontal position, an electric bubble tube is used to detect tilting of the equipment. Description is given now on a conventional type tilt detecting device provided with the electric bubble tube, referring to FIG. 3.
An electric bubble tube 1 comprises a light source 2, a photoelectric conversion element (photodetector element) 3, a photoelectric conversion element (photodetector element) 4, and a bubble tube 5, and tilt is detected according to a signal from the electric bubble tube 1.
The light source 2 is placed under the bubble tube 5 in such a manner that the optical axis of the light source 2 runs perpendicularly to the axis of the bubble tube 5. On the other side of the bubble tube 5 opposite to the light source 2, a pair of photodetector elements 3 and 4 are arranged at symmetrical positions with respect to the optical axis of the light source 2, the two elements being separated from each other by a predetermined spacing.
The light source 2 is driven by a light source driving unit 6, and detection light 7 emitted from the light source 2 passes through the bubble tube 5 and reaches the photodetector elements 3 and 4. Each of the photodetector elements 3 and 4 outputs detected photodetection electric current corresponding to the amount of detected light to a tilt detection control unit 19. The tilt detection control unit 19 comprises a differential amplifier 8 and a control arithmetic unit 9. The differential amplifier 8 amplifies a difference of electric current values of output signals from the photodetector elements 3 and 4, performs current-voltage conversion, and outputs the result to the control arithmetic unit 9. The control arithmetic unit 9 processes the signal from the differential amplifier 8 and calculates a tilt angle.
The detection light 7 emitted from the light source 2 transmits a portion containing only liquid 10 of the bubble tube 5, but, in a portion containing an air bubble 11, it is reflected by boundary surface between the air bubble 11 and the liquid 10, and it is blocked. Therefore, amount of the detection light 7 entering the photodetector elements 3 and 4 varies according to the position of the air bubble 11. Because the position of the air bubble 11 moves according to tilting, tilt is detected by finding a difference between the amount of the light received from the photodetector element 3 and that of the photodetector element 4.
The tilt detecting device as described above is mounted in various types of survey instrument, e.g. a laser survey instrument. In the following, description will be given on general features of a laser survey instrument provided with a tilt detecting device referring to FIG. 4 and FIG. 5.
In a laser survey instrument, a laser beam having directivity is projected in a horizontal direction, and an irradiation plane is formed by rotary irradiation. A photodetection unit (not shown) is positioned on the rotary irradiation plane so as to receive and detect the laser beam and the position of irradiation plane. Thus, a reference line or a reference plane can be obtained.
In the figure, reference numeral 14 represents a laser beam emitter. The laser beam emitter 14 is supported in such a manner that it can be tilted in all directions, and a rotator 15 rotatable around the optical axis of the laser beam emitter 14 is mounted on its head. On the laser beam emitter 14, electric bubble tubes 16 and 17 as described above are mounted, and these bubble tubes run perpendicularly to two different horizontal directions. Further, a vertical tilt sensor 18 is disposed. The tilt sensor 18, the electric bubble tubes 16 and 17, and the tilt detection control unit 19 constitute together a tilt detecting device.
The rotator 15 deflects an emitted laser beam 21 toward a horizontal direction, and it is rotated by a scanning motor 22, thus projecting the laser beam 21 by rotary irradiation.
Arms 23 and 24 (the arm 24 is not shown in the figure) are extended from the laser beam emitter 14 in two different horizontal directions running perpendicularly to each other. Tip of each of the arms 23 and 24 engages with a tilt mechanism.
The tilt mechanism comprises two sets of tilt driving units 25 and 26 (the tilt driving unit 26 is not shown) mounted with respect to the arms 23 and 24 respectively, and a tilt control unit (not shown) for controlling the tilt driving units 25 and 26. Each of the tilt driving units 25 and 26 comprises a screw 27 extending in the direction of the optical axis of the laser beam emitter 14, a nut 28 screwed with the screw 27 and in contact with tip of the arms 23 or 24, and a tilt adjusting motor 32 for turning the screw 27 via gears 30 and 31. In the figure, reference numeral 29 represents a focus adjusting device, which adjusts focal point of the laser beam 21 by moving a condenser lens 33 arranged in an optical path of the laser beam emitter 14 in the direction of the optical axis.
As shown in FIG. 4, the tilt detection control unit 19 comprises the differential amplifier 8 where photodetection signals from the photodetector elements 3 and 4 are inputted, and the control arithmetic unit 9, which issues a control signal based on the signal from the differential amplifier 8. Based on the control signal from the control arithmetic unit 9, a driving circuit 34 drives the tilt adjusting motor 32, and the optical axis of the laser beam emitter 14 is adjusted to a vertical direction or to a predetermined angle.
When the laser beam 21 is projected in a horizontal direction from the rotator 15 and the rotator 15 is rotated by the scanning motor 22, an irradiation plane is formed. By setting the scanning position of the laser beam 21 to a predetermined position using a photodetection device, a reference plane can be obtained.
When the reference plane is obtained, the operating position in wide range can be easily determined. For example, this is applied in the setting of window position in interior operation of construction works or for land grading in civil engineering works.
However, the conventional type tilt detecting device as described above has the following disadvantages:
(1) Because the amount of light emitted from the light source 2 is temperature-dependent, the amount of emitted light varies when ambient temperature changes. This leads to the change of sensitivity in the angle detection and makes it difficult to perform accurate the angle detection.
(2) When external light enters, the difference of light amount entering the photodetector elements 3 and 4 at left and right is offset and is outputted from the differential amplifier 8. This makes it difficult to perform accurate the tilt angle detection.
(3) Because offset and drift of the differential amplifier 8 are outputted as error, the tilt angle detection with high accuracy cannot be achieved. To perform the detection at high accuracy, it is necessary to use an amplifier with smaller values of offset and drift in the differential amplifier 8. This means the use of expensive components and higher cost.
(4) Dark current is present in the photodetector elements 3 and 4, and this emerges in the output as offset of the differential amplifier 8. Because dark current is highly dependent on temperature, output of the detector is changed due to temperature fluctuation, and this leads to inaccurate detection of tilt angle.
(5) It is possible to increase S/N ratio by increasing the amount of the light emitted from the light source 2 and to reduce the influence of drift and offset. However, to avoid the influence of drift and offset caused by the increase of the amount of emitted light, the amount of the light emitted from the light source 2 must be increased to such an extent that the causes of error as described above become negligible, and this requires high driving current. Therefore, when the equipment is incorporated in a portable type device used in outdoor application, service life of batteries is shortened, and this also reduces life of the light source 2.